SURFACE COATING METHOD

Abstract: A surface coating method is provided which can make an anticorrosive coating film having a high corrosion resistance with reduced man-hour and low costs. The method comprises coating a metal substrate with a bake-type metal anticorrosive composition containing a water-soluble chromate compound and zinc powder, baking the coated metal substrate, immediately followed by dipping the baked metal substrate into a liquid compositon either or both of a chromate compound and a resin monomer.

Description: SURFACE COATING METHOD BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a surface coating method, more particularly to a method of forming a coating film having a high corrosion resistance on a metal surface. Description of Related Art Various coating compositions for use in preventing corro¬ sion of a metal such as steel are well-known. For example, a metal anticorrosive composition containing chromic anhydride and a powder of a metal such as zinc and aluminum is disclosed by JP 60-50228(B) as a coating composition exhibiting superior properties. As such a coating composition, Dacrodip (trade¬ mark) is commercially available. This Dacrodip normally supplied to users comprises a com¬ bination of a first component mainly comprised of chromic an¬ hydride and water with a second component comprised of a metal powder and a low-molecular-weight oxohydroxyether such as propylene glycol, and in addition a thickener. When Dacrodip is used, users mix the first component, the second component and a thickener and apply a predetermined amount of the mix¬ ture onto a metal substrate which is to be provided with a coating film. The applied amount may be normally one which is required to provide a thickness of several micrometers. Then, the applied metal substrate may be heated to about 300°C for a predetermined period of time to form a the coating film on the metal substrate. This coating film exhibits an excellent resistance to corrosion by brines, but recently higher resistance has been in demand. In order to meet this demand, the coating treat¬ ment as mentioned above, which is hereinafter referred to as "the first coating step", has often been followed by an addi¬ tional chromating step or coating step, which is referred to as "the second coating step". The second coating step follow¬ ing the first step makes it possible to meet the demanded an- ticorrosion, but increases the number of steps to include nearly two-fold steps, because the first coating step includes coating, baking and cooling and the second coating step also includes coating, baking and cooling. There is such a problem that the treating time and costs are greatly increased. SUMMARY OF THE INVENTION The object of the present invention is to solve the above-mentioned problem and to provide a method of forming an anticorrosive coating film on a metal surface with a reduced number of steps and lower costs. The first aspect of the present invention is a method of surface coating a metal substrate with a bake-type metal anti¬ corrosive composition comprising a water-soluble chromate com¬ pound and zinc powder and baking the coated metal substrate, immediately followed by dipping the baked metal substrate into a liquid composition containing a chromate compound and/or a resin. The second aspect of the present invention is the method of surface coating according to the first aspect, wherein the baked metal substrate is dipped into the above-mentioned liq¬ uid composition, while the substrate is being maintained at a temperature of 50°C or higher. The third aspect of the present invention is the method of surface coating according to the first or second aspect, wherein the dipping is carried out for a period of 30 minutes or less. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The bake-type metal anticorrosive composition used in the present invention should contain a water-soluble chromate com¬ pound and zinc powder. The water-soluble chromate compounds used herein are not specified but all known, for example, they may be chromic acid, a water-soluble metal salt of chromic acid such as cal¬ cium chromate and magnesium chromate and a diσhromate such as zinc dichromate, potassium dichromate, sodium dichromate, mag¬ nesium dichromate and calcium dichromate. The zinc powders used herein may be in any shape. Of these powders, zinc flakes are preferred, particularly prefer¬ ably the flakes have a thickness of 0.1-0.5 μm and a length of up to 15 urn. The zinc powder may be used together with an aluminum powder. The content of the water-soluble chromate compound is in the range of 1-12% by weight, preferably 2-8% by weight, based on the weight of the final composition. The content of the zinc powder is in the range of 10-40% by weight, preferably 15-30% by weight, based on the weight of the final composi¬ tion. The bake-type metal anticorrosive composition contains not only the water-soluble chromate compound and the zinc pow¬ der but also may contain a boric acid or boron oxide, a low- molecular-weight oxohydroxyether and a pH adjuster, a wetting agent, and water and/or an organic solvent, as desired. As the boric acid is preferred commercially available or- thoboric acid. If necessary, etaboric acid or tetraboric acid may be used in place of or in combination with the ortho- boric acid. In the case where boric acid and/or boric oxide is used, the content thereof in the bake-type metal anticorro¬ sive composition is normally in the range of 10-75% by weight, preferably 15-50% by weight, based on the total weight of the boric acid and/or boric oxide and the water-soluble chromate compound. The content of boric acid and/or boric oxide within this range provides the composition with a good anticorrosion in both environments of brine and plain water. The low-molecular-weight oxohydroxyether may be glycol or a low-molecular-weight ether-type polymer thereof. For exam¬ ple, it may be ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, diacetone alcohol or members of similar groups, or mixtures thereof. These ethers can act as a reducing agent for the water-soluble chromate compound to change in the chro¬ mate compound, and make it easy to form a uniform anticorro¬ sive coating film, because the ethers can gradually work while the coating film is being formed by applying and baking the bake-type metal anticorrosive composition, to thereby prevent the boiling of the solvent, etc. The pH adjuster is effectively used when the water-solu¬ ble chromate compound is strongly acidic, and it is used for adjusting the pH of the mixed solution within the range of 3.0-6.0 and normally selected from an oxide and hydroxide of a metal such as lithium or an element of Group IIA and Groups of higher numbers than IIA, such as strontium, calcium, barium, magnesium, zinc, cadmium and etc. The pH adjuster can keep the storage stability of the bake-type metal anticorrosive composition in an optimum state to prevent rapid reaction of the zinc powder and the acids in the liquid, which reaction causes degradation of the adhesion of the coating film and darkening of the appearance of the coating film. A wetting agent may be used for assisting the suspension and dispersion of the zinc powder, and it may be a nonionic surface active agent, particularly an alkylphenol polyethoxy adduct, for example, "Nopco 1592" (trademark) made by Diamond Shamrock Chemical Co. The bake-type metal anticorrosive composition used in the present invention may be prepared by mixing the above-men¬ tioned components by a known process, for example, using a high-speed mixer. As the bake-type metal anticorrosive composition is pre¬ ferred a composition (commercially available under tradename of "Dacrodip") which contains chromic anhydride, a metal such as zinc or aluminum, a pH adjuster of an oxide or hydroxide of a metal, a low-molecular-weight oxohydroxyether such as a polyglycol and a solvent, which is disclosed by JP 60- 50228(B). Normally, this bake-type metal anticorrosive composition is supplied to users in a combined form of a first component mainly comprised of chromic anhydride and water, with a second component comprised of the metal powder and the low-moleσular- weight oxohydroxyether and the thickener. Users mix the first component, the second component and the thickener immediately before use, and then apply a prede¬ termined quantity of the mixture onto a metal -substrate to be coated with the coating film. The applied quantity is nor¬ mally one which is required to finally form the coating film of several micrometers or more. Subsequently, the coated metal substrate is heated to a temperature of about 300°C for a predetermined period of time to thereby form the coating film on the metal substrate. The metal substrate is not limited in the size and shape thereof and already known. The method of the present inven¬ tion is particularly advantageous for preventing the corrosion of steels and hence often used for steel substrates. This method is preferably carried out after dusts or fats are removed from the surfaces of the substrates by an alkali cleaner or a chlorinated solvent. The method of coating the bake-type metal anticorrosive composition on the metal substrates comprises the composition- applying step and the baking step. The applying step may be carried out by a known way, for example, brushing, padding, spraying, hot spraying, air spray¬ ing, electrostatic coating, roller coating, curtain flow coat¬ ing, dip coating, electrodepositing, spatula coating, etc. In the case where the dip coating is carried out, an excess of the composition may be removed by centrifuge or vibration after the dipping. The baking step is carried out by using a hot air circu¬ lation furnace (LPG-burning furnace or electric furnace), far infrared ray heating furnace, infrared ray heating furnace, radiofrequency induction furnace, etc. or combinations of these furnaces. The heat treatment is carried out at a tem¬ perature of 180°C or higher for a period of at least 0.2 sec¬ ond, preferably at a temperature of 200°C or higher for a period of at least o.5 second, more preferably at a tempera¬ ture of 260°C or higher for a period of at least 0.5 second. The coating may be repeated, as desired. The coating film made on the metal substrate is normally 1 μm thick or more, more preferably 3 μ thick or more. This thickness is required to exhibit more stably resistant to corrosion. In the method of the present invention, the coated metal substrate is dipped into a liquid composition -containing a chromate compound and/or a resin immediately after the baking step in the coating process. The terms "dipped, immediately after the baking step" or "baking ..., immediately followed by dipping" used herein means that the baked metal substrate is dipped while it does not completely cool. Thus, the dipping is carried out before the substrate cools completely. The temperature of the sub¬ strate to be dipped may be 50°C or higher, normally 50-350°C, more preferably 100-300°C. If this temperature is within the range of 50-350°C, a beautiful coating film can particularly be formed. The liquid composition containing a chromate compound and/or a resin monomer may be in general one capable of being applied and then dried to form the coating film. The chromate compound contained in the liquid composition may be the same as water-soluble chromate compound contained in the bake-type metal anticorrosive composition. The kind of the chromate compound used in the dipping may be identical with or different from the kind of the water-soluble chromate compound used in the bake-type metal anticorrosive composi¬ tion. The chromate compounds used in the dipping may be used singly or in combination. As the resin may be made reference to a water-soluble resin, a resin in aqueous emulsion, or a resin in aqueous sus¬ pension. Examples of the resins are an acryl resin, polyester resin, polyethylene resin, polyvinyl resin, epoxy resin, buta¬ diene resin, melamine resin, alkyd resin, phenol resin, etc. Of these resins is preferred the acryl resin. The liquid composition may contain the chromate compound without containing the resin, contain the resin without con¬ taining the chromate compound, or contain both the chromate compound and the resin. In the present invention, the liquid composition containing both chromate compound and resin is preferred. The liquid composition containing both chromate compound and resin normally contains 0.1-50% by weight, preferably 0.5-30% by weight, of the chromate compound and normally 0.5-40% by weight, preferably 0.5-20% by weight, of the resin. As the liquid composition may be preferably made refer¬ ence to a chromation liquid commercially available as "Dacromet #100" made by Nippon Dacro Shamrock K.K. and a resin coating agent commercially available as "Dacromet LTX" made by Nippon Dacro Shamrock K.K. In the case where the liquid composition contains a solid content, the solid content thereof is preferably adjusted to 0.5-90% by weight, more preferably to 1-50% by weight. If the solid content is in the range of 0.5-90% by weight, a coating film having a beautiful appearance and a proper thickness can be obtained. In the surface treating method according to the present invention, the temperature of the liquid composition may be important. If the temperature of the liquid composition is excessively low, the cooling effect is too high to properly form the coating film. If the temperature of the liquid com¬ position is excessively high, on the other hand, the liquid composition itself may be degraded. The temperature of the liquid composition may vary depending upon the temperature of the metal substrate, normally preferred to be adjusted to the range of 5-95°C. The metal substrate baked in the coating step is dipped in the liquid composition preferably for 20 minutes or less, normally 0.1 second to 30 minutes, preferably 0.5 second to 5 minutes. If the dipping time is in the range of 0.1 second to 30 minutes, the coating film can fully be formed on the metal substrate and, furthermore, the temperature of the substrate is not excessively lowered and hence the drying effect by the baking of the coating step is not lost. According to the present invention, no baking is normally needed after the dipping step. However, a simple drying may be carried out, as desired. The dipping step forms a coating film of normally 0.1 μm thick or more, preferably 1 μm thick or more. The coating film of 1 μm thick or more is preferred, since it makes it possible to afford a more stable corrosion resistance to the coating film made by the first coating step. The present invention will be illustrated below with ref¬ erence to some examples, but it is not limited to these exam¬ ples. Evaluation of the coating films fabricated in the exam¬ ples was made in the following manner: 1. Appearance Thinness and unevenness of the coating films were visu¬ ally observed. Evaluation is classified in three criteria: V Very good G Good P Poor G-P Somewhat poor but no practical problem 2. Anticorrosion Performance After a composite cyclic corrosion test was conducted, occurrence of red rust was visually observed. Evaluation is classified in three criteria: V Very good G Good P Poor G-P Somewhat poor but no practical problems 3. Time Required for Working Time required for working (man-hour) was evaluated rela¬ tively to Comparative Example 1 as a basis. A bolt of M 10 mild steel (entire length: about 47 mm; weight: about 22 grams) which was washed with a solvent vapor and dry honed was used as a test substrate. Example 1 In the coating step was used Dacrodip (trademark) as the bake-type metal anticorrosive composition, which was made operable by mixing in accordance with the recipe. In the dip¬ ping step was used Dacromet #100BL (trademark; black chromat- ing liquid) as the chromate compound-containing liquid compo¬ sition, which was free of a resin. The composition of Dacromet is as follows: Chromic acid about 4% by weight Zinc powder about 20% by weight The test substrate was dipped into Dacrodip, and an ex¬ cess of Dacrodip on the substrate was centrifugally removed. In the baking stage, the test substrate was placed in an elec¬ trically-heated hot air circulation oven, heated until the test substrate reached 300°C and maintained at the same tern- perature for 5 minutes. When the test substrate removed from the oven cooled to 290°C, it was dipped into Dacromet #100BL regulated to 50°C, pulled up in 1 second and dried. The quantity of the coating film adhered to the test sub¬ strate in the coating step was 100 mg/dm 2 and the quantity ad¬ hered in the dipping step was 50 mg/dm 2 The dipping-finished test substrate was evaluated in the above-mentioned manner. The results are shown in Table 1. Example 2 The same procedures as in Example 1 were repeated, except the following: As the liquid composition for dipping was used Dacromet LTX (trademark; clear) containing a resin but free of a chro¬ mate compound. After the baking stage of the coating step, the test substrate was dipped into Dacromet LTX (trademark; clear) when it cooled to 250°C, and then pulled up in 2 sec¬ onds and dried. The quantity of the coating film adhered to the test sub¬ strate by the coating step was 100 mg/dm 2 and the quantity ad¬ hered in the dipping step was 80 mg/dm 2 . The dipping-finished test substrate was evaluated in the above-mentioned manner. The results are shown in Table 1. Example 3 The same procedures as in Example 1 were repeated, except for the following: The liquid composition used in the dipping step was Dacromet #100(trademark; a chromating liquid) containing a chromate compound and free of a resin. The dipping time was 5 seconds. The quantity of the coating film adhered to the test sub¬ strate was 100 mg/dm 2 , and the quantity adhered in the dipping step was 50 mg/dm 2 . The dipping-finished test substrate was evaluated in the above-mentioned manner. The results are shown in Table 1. Example 4 The same procedures as in Example 1 were repeated, except for the following: Baking in the coating step was conducted by heating the test substrate to 350°C in an electrically heated hot air cir¬ culation oven and the maintaining the substrate at the same temperature for 5 minutes. When the substrate removed from the oven cooled to 300°C, it was dipped into the liquid compo¬ sition adjusted to 95°C and pulled up in 5 seconds and dried. The quantity of the coating film adhered to the test sub¬ strate was 100 mg/dm 2 , and the quantity adhered in the dipping step was 100 mg/dm 2 . The dipping-finished test substrate was evaluated in the above-mentioned manner. The results are shown in Table 1. Example 5 The same procedures as in Example 1 were repeated, except for the following: When a test substrate baked in the coating step and then removed from an electrically heated hot air circulation oven cooled to 270°C, it was dipped into the liquid composition ad¬ justed to 20°C, and then pulled up in 0.5 seconds and dried. The quantity of the coating film adhered to the test sub¬ strate in the coating step was 100 mg/dm 2 , and the quantity adhered in the dipping step was 30 mg/dm 2 . The dipping-fin¬ ished test substrate was evaluated in the above-mentioned man¬ ner. The results are shown in Table 1. Example 6 The same procedures as in Example 1 were repeated, except for the coating step was twice repeated, followed by the dip¬ ping step. The quantity of the coating film adhered -to the test sub¬ strate in the coating step was 200 mg/dm 2 , and the quantity adhered in the dipping step was 100 mg/dm 2 . The dipping-fin¬ ished test substrate was evaluated in the above-mentioned man¬ ner. The results are shown in Table 1. Example 7 The same procedures as in Example 1 were repeated, except for the following: As the liquid composition in the dipping step was used Dacromet #100BL (trademark; σhromating liquid) containing an aqueous 10% acrylic ester as resin (made by Showa Kobunshi K.K.) added thereto. The quantity of the coating film adhered to the test sub¬ strate in the coating step was 100 mg/dm 2 , and the quantity adhered in the dipping step was 50 mg/dm 2 . The dipping-fin¬ ished test substrate was evaluated in the above-mentioned man¬ ner. The result are shown in Table 1. Comparative Example 1 In the first coating step was used Dacrodip as the bake- type metal anticorrosive composition, which was made operable by mixing in accordance with the recipe. In the second coat¬ ing step was used Dacromet #100BL (black chro ation liquid) as the liquid composition containing a chromate compound and/or resin monomer. A test substrate was dipped in the Dacrodip, and an ex¬ cess of the Dacrodip on the substrate was centrifugally re¬ moved. Then, the test substrate was baked by heating in an electrically heated hot air circulation oven to 300°C and maintaining at the same temperature for 5 minutes. The test substrate removed from the oven was allowed to cool to room temperature. In the same manner as in the first coating step, the test substrate was dipped in Dacromet #100BL as the liquid composi¬ tion, and an excess of the liquid composition on the substrate was centrifugally removed. The test substrate was again baked by heating in the electrically heated hot air circulation oven to 250°C and maintaining at the same temperature for 5 min¬ utes. The quantity of the coating film adhered to the test sub¬ strate in the first coating step was 100 mg/dm 2 , and the quan¬ tity adhered in the second coating step was 50 mg/dm 2 . The coating-finished test substrate was evaluated in the above- mentioned manner. The results are shown in Table 1. Comparative Example 2 The same procedures as in Comparative Example 1 were repeated, except for the following: In the second coating step was used Dacromet LTX (clear), which is a resin coating agent. The baking in the second coating step was conducted by heating the test substrate to 100°C and maintaining it at the same temperature for 5 minutes in the electrically heated hot air circulation oven. The test substrate removed from the oven was allowed to cool to room temperature. In the first coating step, the quantity of the coating film adhered to the test substrate was 100 mg/dm 2 . The quan¬ tity adhered in the second coating step was 80 mg/dm 2 . The coating-finished test substrate was evaluated in the above- mentioned manner. The results are shown in Table 1. Comparative Example 3 The same procedures as in Example 1 were repeated, except that the second coating step was not carried out. The quantity of the coating film adhered to the test sub¬ strate was 100 mg/dm 2 . The coating-finished test substrate was evaluated in the above-mentioned manner. The results are shown in Table 1. Comparative Example 4 The same coating step as in Example 6 was twice repeated, except that the second step was not carried out. The quantity of the coating film adhered to the test sub¬ strate was 200 mg/dm2. The coating-finished test substrate was evaluated in the above-mentioned manner. The results are shown in Table 1. 13 Table 1 Appearance Anticorrosion Time Required Performance For Working Example 1 V G 6 Example 2 V G-P 6 Example 3 V G 6 Example 4 V G 6 Example 5 V G 6 Example 6 V V 11 Example 7 V G 6 Comp. Exam. 1 P G-P 10 Comp. Exam. 2 P G-P 9 Comp. Exam. 3 P P 5 Comp. Exam. 4 G G 10 Conventional surface coating methods require a drying apparatus in the dipping step and thus require much man-hour and operating time. On the other hand, the surface coating method of the present invention can eliminate the drying step in the coating step by utilizing the high temperature of the metal substrate in the baking of the coating step. Therefore, the surface coating method of the present invention can sim¬ plify the apparatuses and shorten the operating time. Therefore, the method of the present invention is not only effective in industries and contribute to saving of energies and environmental protection.

Claims: WHAT IS CLAIMED IS: 1. A surface coating method which comprises coating a metal substrate on the surface thereof with a bake-type metal anti¬ corrosive composition containing a water-soluble chromate com¬ pound and a zinc powder, baking the coated metal substrate, immediately followed by dipping the baked metal substrate into a liquid composition containing either or both of a chromate compound and a resin. 2. The surface coating method according to Claim 1, wherein said surface coated metal substrate is dipped in said liquid composition while said substrate being maintained at a temper¬ ature of 50°C or higher. 3. The surface coating method according to Claim 1, wherein the metal substrate is made of steel. 4. The surface coating method according to Claim 1, wherein the water-soluble chromate compound is chromic acid or a water-soluble metal salt of chromic acid such as calcium chro¬ mate, magnesium chromate, zinc dichromate, potassium dichro¬ mate, sodium dichromate, magnesium dichromate and calcium dichromate. 5. The surface coating method according to Claim 1, wherein the zinc powder is in a flake form. 6. The surface coating method according to Claim 1, wherein the flake form has a thickness of 0.1-0.5 μm and a length of up to 15 μm. 7. The surface coating method according to Claim 1, wherein the zinc powder is used together with an aluminum powder. 8. The surface coating method according to Claim 1, wherein the content of the water-soluble chromate is in the range of 1-12% by weight of the composition. 9. The surface coating method according to Claim 1, wherein the content of the water-soluble chromate is in the range of 1-8% by weight of the composition. 10. The surface coating method according to Claim 1, wherein the content of the zinc powder is in the range of 10-40% by weight of the composition. 11. The surface coating method according to Claim 1, wherein the content of the zinc powder is in the range of 15-30% by weight of the composition. 12. The surface coating method according to Claim 1, wherein the bake-type metal anticorrosive composition further contains a boric acid or boric oxide, a low-molecular-weight oxohydrox¬ yether, a pH adjuster, a wetting agent, and/or an organic sol¬ vent. 13. The surface coating method according to Claim 12, wherein the boric acid is orthoboric acid, metaboric acid and/or tetraboric acid. 14. The surface coating method according to Claim 12, wherein the low-molecular-weight oxohydroxyether is glycol or a low- molecular-weight ether-type polymer thereof. 15. The surface coating method according to Claim 14, wherein the low-molecular-weight oxohydroxyether is ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, tri- ethylene glycol, tripropylene glycol, diacetone alcohol or members of the similar groups, or mixtures thereof. 16. The surface coating method according to Claim 1, wherein the baked metal substrate is dipped in said liquid composition for 30 minutes or less. 17. The surface coating method according to Claim 2, wherein the baked metal substrate is dipped in said liquid composition for 30 minutes or less.

Inventors: NAKAZATO MICHIAKI, SUGANO KAZUAKI, NAKAZATO, MICHIAKI, SUGANO, KAZUAKI
Applicants: NAKAZATO MICHIAKI, NIPPON DACRO SHAMROCK CO, SUGANO KAZUAKI, NAKAZATO, MICHIAKI, NIPPON DACRO SHAMROCK CO., SUGANO, KAZUAKI
Application year: 1994
Main classification: C23C22
IPC codes: C07K14, C09D5, C23C22